1. Field of the Invention
This invention relates to bidirectional communication between a single master apparatus and a plurality of slave apparatus and, more particularly but not exclusively, to a method and apparatus for communication between video information apparatus, such as VTR (video tape recorder) or video disc player, and peripheral apparatus, such as a tuner, a timer, a video camera, editing apparatus, a computer and so on.
2. Description of the Prior Art
Recently, in order to enhance the enjoyment of video equipment, a video information system has been developed, which comprises a VTR as master apparatus and a plurality of peripheral or slave apparatus, such as camera, a television tuner unit, a timer unit and apparatus. In such a system, the communication of control data, such as a mode signal and various other control signals, between the VTR and each peripheral apparatus becomes a significant problem.
In a system for bidirectional communication between a VTR used as master apparatus and a plurality of peripheral apparatus each used as a slave apparatus, it is important that the number of communication lines be reduced as much as possible. This becomes increasingly important as the number of slave apparatus is increased.
A bidirectional communication system having at the side of the master apparatus only two communication lines one of which is a transmission communication line and the other of which is a reception communication line has much to recommend it. Alternatively, a bidirectional communication system having only one communication line may be employed.
The assignee of the present application has previously proposed a bidirectional communication system using only one communication line (see copending U.S. patent application Ser. No. 06/732,004, filed May 6, 1985, and assigned to the assignee of the present application).
In this previously proposed system, a communication interval formed of a plurality of master and slave transmission areas that are arranged in a time division manner is taken as one block, and this block is repeated cyclically. The time control for communication is provided exclusively by the master apparatus. The number of slave apparatus transmission areas is equal to the number of slave apparatus.
Even when two communication lines (master apparatus transmission line and slave apparatus transmission line) are used, seen from the side of the slave apparatus the transmission lines appear as one. Accordingly, when the communication is carried out cyclically as described above, the transmissions from the several slave apparatus are effected in the respective transmission areas in a time division manner.
However, since transmission areas are provided for all of the plurality of slave apparatus in a time division manner, as the number of slave apparatus increases, the total transmission period becomes very long.
Since each slave apparatus does not always request the master apparatus to transmit video information, if a transmission area is provided for each of the plurality of slave apparatus as described above, an unoccupied or useless transmission area occurs, resulting in low time efficiency.
On the other hand, it frequently happens that a mode signal, a control signal and a command signal are communicated between the video information apparatus and the peripheral apparatus, for example, between the VTR and the video camera and the editinq apparatus, in order that the two apparatus may be operated in synchronism with each other.
In the prior art, a communication of this kind is performed regardless of its time relation to an input signal or a reproduced video signal. For example, when a mode is changed, commands such as "playback mode next", "stop mode next" and so on are transmitted and received.
Particularly, when a communication is carried out only in a necessary case, as when the mode is changed as described above, if a miscommunication or erroneous communication occurs, the present mode remains erroneous at least until the next communication is made for changing the mode.
Further, since it is not known in advance exactly when the transmission data will arrive at the peripheral apparatus, the peripheral apparatus must be always in an interruptible state and always in a communicable state, so that the software therefor is difficult to write.
As to the video information apparatus, since it is not known in advance exactly when the command or the like will arrive from the peripheral apparatus, the software therefor is also difficult to write.
For this reason, it may be concluded that the communication should be carried out cyclically. However, if the cycle or period is free-running and not synchronized with a video signal or in a fixed time relation to the latter, various problems occur.
For example, it has recently been proposed that data such as a time code including a frame number, a field number and the like be multiplexed on a video signal and then recorded. In such case, when the time codes are communicated, if the communication between the VTR and the peripheral apparatus is carried out over a period longer than, for example, the vertical period, the communication surpasses the frame number or field number of the time code at a certain time, producing a dropout in the time code output.
Furthermore, if the period of the communication is not synchronized with the video signal, when the command (for example) "set recording mode two fields from now" is issued in a communication between the VTR and the editing apparatus, the time "two fields from now" cannot be determined uniquely. This makes the software therefor very difficult to write.